The present disclosure relates to a wiring board on which electronic parts are mounted and an electronic apparatus which includes the wiring board.
Generally, it is known that, in a printed wiring board, a resonance mode between a ground electrode and a power supply electrode caused by simultaneous switching noise (SSN) has a bad influence on signal integrity and a characteristic of electromagnetic interference (EMI). Thus, various proposals have been made for a technique for reducing the influence of the noise. For example, a technique of disposing a decoupling capacitor on a printed wiring board, another technique of separating planes (electrodes) and so forth have been proposed. Recently, a further technique has been proposed wherein an electromagnetic band gap (EBG) structure which generates an attenuation characteristic (band gap) in a specific frequency band is provided on a printed wiring board. The technique just described is disclosed, for example, in U.S. Published Application No. 2005/0029632 A1 (hereinafter referred to as Patent Document 1) and U.S. Published Application No. 2006/0050010 A1 (hereinafter referred to as Patent Document 2).
In Patent Document 1, an EBG structure of a type called mushroom type is proposed. The mushroom type EBG structure includes a conductor plane of a power supply level, another conductor plane of a ground level, and a conductor layer formed from a plurality of small conductor pieces of a polygonal surface shape, the conductor layer being provided between the two conductor planes and disposed periodically in arrays in the board surface. Further, in the EBG structure of the mushroom type, the small conductor pieces are electrically connected to one of the conductor plane of the power supply level and the conductor plane of the ground level. Furthermore, in the EBG structure of Patent Document 1, a dielectric layer is provided between the conductor plane of the power supply level and the conductor layer and between the conductor plane of the ground level and the conductor layer.
The EBG structure of the mushroom type proposed in Patent Document 1 has a three-layer structure wherein three conductive films are stacked with the dielectric layers interposed therebetween. Therefore, in the technology of Patent Document 1, in order to provide the EBG structure on a printed wiring board, it is necessary to additionally provide one layer of a conductive film between the conductor plane of the power supply level and the conductor plane of the ground level. In this instance, there is a subject that the structure of the printed wiring board is complicated and a higher cost is required for the printed wiring board.
In order to solve the subject of the EBG structure of the mushroom type described above, Patent Document 2 proposes an EBG structure configured such that there is no necessity to provide one layer of a conductive film between the conductor plane of the power supply level and the conductor plane of the ground level. The EBG structure proposed in Patent Document 2 is called AI (Alternating Impedance)-EBG structure.
In the AI-EBG structure, one of the conductive film connected to the power supply and the grounded conductor film is configured from a plurality of small conductor pieces (hereinafter referred to as patch electrode portions) of a square surface shape of a large size, and a plurality of small conductor pieces (hereinafter referred to as branch electrode portions) of a square surface shape of a small size. In the AI-EBG structure, the patch electrode portions are periodically arrayed on the board surface, and each of the branch electrode portions is disposed so as to electrically connect two adjacent ones of the patch electrode portions. Such an AI-EBG structure as just described is a two-layer structure wherein two conductive films are stacked with a dielectric layer interposed therebetween.
It is to be noted that both of the EBG structure of the mushroom type and the AI-EBG structure described above act as a filter for suppressing noise in a desired frequency band. Therefore, in the EBG structures, the size and the shape of the conductor elements, namely, the small conductor pieces and the patch electrode portions, are adjusted to suitably set the capacitance component and the inductance component of the EBG structure thereby to cause a band gap, namely, an attenuation band, to appear in a predetermined frequency band.
If it is tried to apply the EBG structure described above to consumer applications such as, for example, a portable telephone set, then there are two subjects including increase of the frequency band in which noise is suppressed and miniaturization of the EBG structure. Particularly with regard to the former subject, it is desirable to configure the EBG structure so that the noise suppression effect can be obtained from a comparatively low frequency band.
As a technique for expanding the frequency band in which noise is suppressed, namely, the frequency band of the band gap, in the EBG structure, various technique have been proposed, for example, in JP-T-2010-519777 (hereinafter referred to as Patent Document 3) or Japanese Patent Laid-Open No. 2008-177363 (hereinafter referred to as Patent Document 4). In Patent Document 3, a technique of combining AI-EBG structures having frequency bands of the band gap different from each other is proposed. However, according to the technique of Patent Document 3, since a plurality of AI-EBG structures are provided, miniaturization of the EBG structure and hence of the printed wiring board is difficult. It is to be noted that Patent Document 3 proposes nothing of miniaturization of the EBG structure.
Meanwhile, Patent Document 4 suggests that, by reducing the thickness of a dielectric layer or insulating layer to 25 μm or less, the band gap disappears and the attenuation band can be expanded. However, also data contrary to the suggestion of Patent Document 4 has been disclosed, for example, in Hirotaka TOYODA and four others, “Miniaturization of a planar EBG structure for unnecessary electromagnetic wave propagation suppression formed in power supply/ground planes of printed circuit board,” Journal of the Institute of Electronics, Information and Communication Engineers B, Vol. 10 J90-B, No. 11, pp. 1135-1142 (hereinafter referred to as Non-Patent Document 1). In Non-Patent Document 1, data is disclosed that, in the case where the relative dielectric constant ∈r of the dielectric film provided between the conductor plane of the power supply level and the conductor plane of the ground level is 30 and the film thickness of the dielectric film is 16 μm, a band gap appears from 1.8 GHz.
Also for a technique for miniaturization of the EBG structure or filter, various proposals have been made and are disclosed, for example, in Non-Patent Document 1 or Japanese Patent No. 4755966 (hereinafter referred to as Patent Document 5). In Non-Patent Document 1, a technique of increasing the relative dielectric constant ∈r of a dielectric film to increase the capacitor component of the EBG structure thereby to achieve miniaturization of the EBG structure is proposed. Meanwhile, in Non-Patent Document 1 and Patent Document 5, a technique of providing a magnetic layer on one surface of a dielectric layer to increase the inductance component of the EBG structure thereby to achieve miniaturization of the EBG structure is disposed. However, even if the techniques are used, the size of the EBG structure or filter is a several cm order size, and it is difficult to achieve a several mm order size necessary for consumer applications.
Meanwhile, with regard to miniaturization of the EBG structure, it is suggested that the EBG structure of the mushroom type has a limitation to reduction in thickness of the dielectric layer due to irregularities of the small conductor pieces which configure the EBG structure (refer to, for example, PCT Patent Publication No. WO2009/131140 A1 (hereinafter referred to as Patent Document 6)). Patent Document 6 further suggests that, in order to miniaturize the EBG structure, it is effective to make the distance between a plurality of small conductor pieces and a conductor plate opposing to the small conductor pieces small and to form the dielectric layer from a material having a high relative dielectric constant. However, also it is pointed out in Patent Document 6 that particular measures for implementing the EBG structure of such a configuration as just described have not been found as yet and desired miniaturization of the EBG structure cannot be achieved.
Further, it is proposed in Patent Document 6 that, since it is difficult to form an EBG structure or filter of the several mm order on a printed wiring board, an EBG structure is formed on a rigid board such as a silicon interposer board to fabricate a filter of the several mm order. However, this technique increases the fabrication cost in comparison with an ordinary fabrication process of a printed wiring board. Further, this technique requires, midway of the fabrication process, a step of embedding a rigid board as a part into the printed wiring board, which further increases the fabrication cost. It is to be noted that, in the EBG structure proposed in Patent Document 6, a band gap appears in a predetermined frequency band and the subject of increase of the attenuation band described hereinabove, namely, achievement of a wider bandwidth, is not eliminated.